For the above-mentioned studies materials of two different types have been used, one type being particles which are not biologically degradable and the other type being particles which are biologically degradable i.e. which are degraded in vivo by enzymes occurring in the organisms. Usually the biologically degradable particles have been used in human experiments and the biologically non-degradable in animal experiments. One disadvantage common to these particles has been the problem with leakage of the radionuclide. To prevent this leakage attempts have been made to label the non-degradable particles already in connection with the production thereof so as to get an encapsulation of the radionuclide. These particles, which are made by i.a. different plastic materials, like carbonized ion-exchangers, have, however, several great disadvantages. They have a high density, which may cause sedimentation phenomena in the blood vessels. Also other rheologic disturbances may be caused by the high density, whereby the distribution of the particles in the blood capillaries may diverge in relation to the normal blood flow. Moreover, plastic particles in serum often aggregate, whereby further disorders may arise.
As the labelling with radionuclide is made in connection with the production of the carrier material, the product must be used immediately as radionuclides with a short half-life (5-60 days) are used almost exclusively within this field. The production of microspheres labelled with a radionuclide is very expensive and if it is not possible to use the material within a limited time after the production so that the radionuclide has decayed, great economic losses may occur. This will also limit the use of suitable radionuclides.
The degradable particles are usually labelled just before use. Hereby it has been possible to tolerate radionuclide labellings with low stability partly because of the fact that the microspheres have a limited in vivo life-time (due to the enzymatic degradation), partly because of the fact that generally radionuclides with a very short half-life have been used, which means that the choice of suitable radionuclides will be very limited. To make the use of more long-lived radionuclides possible without risking a deposition thereof in different organs, a stable and leakage-free binding of the radionuclide to the particles and also to the particle fragments obtained by the enzymatic degradation must be effected.
Also many of the proposed degradable particles, i.e. those based on denatured proteins, like albumin, have too high density and/or adhesitivity and may cause sedimentation and/or aggregation of the particles in the suspension. However, the particles known from Swedish patent application No. 7407461-8 do not have these mechanical disadvantages.